The invention relates generally to a radiation treatment apparatus, and more particularly to such apparatus employing a linear accelerator to provide X-rays or other particle beams for therapeutic medical applications. Such linear accelerators typically have an injection point where particles originate in the accelerator, an insertion point where RF energy enters the accelerator, and an exit point from which the particles are discharged from the accelerator. The particles may be used directly for treatment or converted to X-rays by striking a target, typically made from a high density material such as gold.
The use of linear accelerators in radiation therapy is generally known. Linear accelerators are used for generating a high energy radiation beam to be directed at tissue for treatment. As is well-known, a typical radiation therapy apparatus includes a stand anchored firmly to the floor of a room and a gantry rotatable on a bearing in the stand. The operational accelerator structure, housed within and oriented substantially parallel to a cantilevered strut section of the gantry, is rotatable with the gantry about the bearing to enable the treatment head at the remote end of the strut section to be positioned in a continuum of positions and orientations around a patient or object situated on a platform at the isocenter of the apparatus.
While such radiation therapy systems have been very successful, a problem has arisen in radiation therapy systems employing cantilevered linear accelerators. When the gantry is oriented at specific angular positions/orientations with respect to the stand, the accelerated particle beam may become slightly misdirected with respect to the target, producing potentially unsatisfactory results. More particularly, depending upon the angular position of the gantry, the cantilevered strut section of the gantry and the similarly cantilevered linear accelerator disposed therein differentially deflect. This differential deflection will cause the particle beam to follow a different path within the bending magnet and to strike the target at different input angles, ultimately affecting the X-ray beam intensity profile of the radiated X-rays as the gantry is rotated. The flatness of the beam intensity profile at all gantry angles is referred to as "rotational flatness".
Attempts have been made to electronically correct the misalignment problem attributable to gravitational deflection, but such efforts, standing alone, have not been entirely effective. In particular, such electronic solutions must generally include circuitry to measure the amount of misalignment and incorporate a feedback mechanism to correct for the misalignment. An example of an appropriate feedback mechanism is one or more wound coils disposed proximately to the beam path for directing the beam path. This type of solution is considerably more complex than the invention disclosed herein, and fails to likewise address the cause of the misalignment problem.
Additional attempts have been made to improve rotational flatness by stiffening the support plate used to affix the linear waveguide accelerator to the cantilevered gantry section. However, such methods for resolving the rotational flatness problem have been relatively ineffective because they offer no ready means of adjustment.